Liquid crystal displays are in wide use as display sections in, for example, portable apparatuses which are required to be small and power-saving. FIG. 3 shows, as an example, an arrangement of such a liquid crystal display.
Referring to FIG. 3, the active matrix liquid crystal display 101 has pixel electrodes 16 arranged in a matrix form. Each pixel electrode 16 is connected to its own signal line 18 and scan line 19 via a TFT (thin film transistor) 17 and other active elements. The signal lines 18 and the scan lines 19 are disposed on a first transparent substrate 20. Opposite to the first transparent substrate 20 is disposed a second transparent substrate (not shown) with opposite electrodes (not shown). Liquid crystal (not shown) is sandwiched and sealed between the first transparent substrate 20 and the second transparent substrate.
The active matrix liquid crystal display 101 receives image signals (R0, etc.) from an image signal supply circuit 3. After being adjusted in terms of timing by, for example, a latch circuit 13, the image signals are fed to the signal line drive circuit 111 which in turn feeds signal line drive signals to drive the signal lines 18. A scan line drive circuit 15 feeds scan signals to the scan lines 19 in synchronism with the image signals for vertical scanning of the scan lines 19.
Configured this way, the active matrix liquid crystal display 101 boasts superior image quality and is used in a portable apparatus where high image quality is essential. Demand is strong for portable apparatuses which display with further improved quality and at the same time offer more running hours between battery recharging. To this end, the image display device in a portable apparatus needs to be low in power consumption. The active matrix liquid crystal display 101 is a liquid crystal display and inherently consumes relatively small amounts of electric power; it is, however, required to improve on the feature to respond to the market demand.
Conventionally, the large majority of active matrix liquid crystal displays 101 has been transmissive. However, reflective and reflective/transmissive types are increasingly popular in new portable apparatuses, especially very compact apparatuses like portable telephones. This is made possible because of the development of reflective and reflective/transmissive active matrix liquid crystal displays with faithful color reproduction capability. Another reason is that these types of active matrix liquid crystal displays either do not need a backlight at all as transmissive types or uses only a supplementary backlight, saving greatly on the electric power supplied to the backlight.
The signal line drive circuit 111 feeding signal line drive signals to the signal lines 18 ranks right after the backlight in the decreasing order of power consumption. Power saving in the signal line drive circuit 111 is therefore particularly important in the reflective or reflective/transmissive active matrix liquid crystal display 101.
Japanese Examined Patent Publication No. 3007745 (published on Feb. 7, 2000) discloses an invention with an objective to reduce the power consumption in the signal line drive circuit 111. The invention adjusts the position of a buffer circuit in the signal line drive circuit 111. The following will describe the arrangement of the signal line drive circuit 111 in reference to FIG. 4 showing the circuit.
112 represents input terminals where the active matrix liquid crystal display 101 receives image signals. In FIG. 4, the image signals are divided into red (R), green (G), and blue (B), 6 bits for each color, and denoted by R0–R5, G0–G5, and B0–B5. 113 represents a sampling and latch circuit that samples and latches the image signals to produce output signals controlling the decoder circuit 114 in the succeeding stage. The decoder circuit 114 converts the image signals to signals controlling reference voltage chooser circuits 115 in the succeeding stage using a decoder table on the basis of the tones represented by the image signals sampled by the sampling and latch circuit 113. The reference voltage chooser circuit 115 chooses one of incoming reference voltages according to the output of the decoder circuit 114.
116 is a voltage divider circuit in which ladder resistors 36, etc. and divides a first reference voltage VB1 fed from an external reference power supply circuit 12. The reference voltages produced by the voltage divider circuit 116 by voltage division will be referred to as the second reference voltages VB2. The first reference voltage VB1 and the second reference voltages VB2 are fed via buffer circuits 117 each having a high input impedance and a low output impedance to each reference voltage chooser circuit 115 where one of the reference voltages is chosen. The output of each reference voltage chooser circuit 115 is transmitted via an output buffer circuit 118 to an output terminal 119 of the signal line drive circuit 111. Arranged in this manner, the signal line drive circuit 111 can save the overall power consumption by reducing the current flow through the voltage divider circuit 116.
The signal line drive circuit 111 in conventional active matrix liquid crystal displays 101 however has reduced its power consumption only by reducing the current flow through some circuits in the signal line drive circuit 111. More tinkering is necessary to achieve sufficient power saving and hence extend running hours of the portable apparatus. Saving on power in the signal line drive circuit 111 contributes a lot to saving on power in the entire image display device. This is especially true with reflective and reflective/transmissive displays, since in these types of displays a highly power-consuming backlight is either unnecessary or used only for supplementary purposes.
Portable telephones, which have gained growing popularity in recent years and are certainly going to enjoy more in the foreseeable future, consume large amounts of power during communication, but little power during standby: the difference in power consumption is well more than 100 times. Accordingly, the required level of power saving varies greatly depending on the operating conditions of the device. Taking a typical portable telephone as an example, the overall power consumption is about 5 mW during standby and about 900 mW during communication. Accordingly, the required level of power saving varies greatly depending on the operating condition of the display in the portable telephone.